As an engineer, I'm skeptical that the Model S, in it's present hardware configuration, can have semi-autonomous driving by this summer. There are simply not enough sensors in the present build for the kind of situational awareness required for it. Semi-autonomous driving is provided as a result of the sum of the data fed back from the sensors, and the collective data are analyzed thousands of times per second and tied back to the power drive unit, steering, friction braking, regen braking, and traction and stability control. The current vintage of Model S autopilot cars (Q4 2014 to Q1 2015) have one front radar, one front monocular Mobileye optical camera, and a 360-degree set of ultrasonic sensors. This present configuration works great for TACC and collision warning, but I believe still has many sensor blind spots for total sensing and active maneuvering required of autonomous driving. (On the other hand, I want to make the distinction that it would be able to Auto-steer.)
Mercedes-Benz have been at it since 1998, so I think their autopilot technology is the standard by which Tesla can be measured by. Here's how the Model S stacks up (below). Audi, Infiniti, Toyota are also realizing that these are the minimum set of eyes for semiautonomous driving. I hope my current Model S is future-proof to accommodate these systems as upgrades, much like how Mobileye can be installed and programmed off-the-shelf.
I'm also optimistic that Tesla can quickly pivot their builds and are presently working on this hardware setup. Tesla's prime advantage over Mercedes-Benz and other ICE car companies is that electric motors have much finer responses over combustion engines for situations requiring very precise wheel response. The characteristic lag response in gas engines can add precious milliseconds when time matters. Tesla can also update software at will. Digital > Analog.
I would like to hear the point of view of 2014 S550 owners that are using this technology.
The illustrations below are for the Mercedes Benz system.
(click for larger images)
keywords: TACC, autopilot, autonomous driving, semiautonomous driving, autosteer, auto-steer, autonomous, semi-autonomous
UPDATE: Elon did not say autonomous or semi-autonomous driving by this summer. He said Auto-steer. Clear distinction there technically. Inaccurately, the media is parsing semantics again for sensational headlines, or they just don't understand.